Isotope analysis of crystalline impact melt rocks from Apollo 16 stations 11 and 13, North Ray Crater

Abstract

Rubidium‐strontium and Sm‐Nd isotopic data were obtained for lunar impact melt rocks from Apollo 16 stations 11 and 13 as part of the North Ray Crater Target Rock Consortium study. Rubidium‐strontium whole‐rock data define an isochron age, T, = 3.69 ± 0.16 b.y. [λ = 0.0142 (b.y.)−1] and initial 87Sr/86Sr, ISr, = 0.69918±5 for feldspathic microporphyritic (FM) samples. Internal Rb‐Sr isochrons of T = 3.86 ± 0.05 b.y., ISr = 0.69952 ± 10 and T = 3.76 ± 0.04 b.y., ISr = 0.69916 ± 4 were determined for very high alumina (VHA) melt rock 67747 and anorthositic noritic melt rock (ANMR) 67559, respectively. (T,ISr) for these and other lunar melt rocks were examined to infer the nature of the protolith for the rocks. ISr values for the AN MR and FM rocks are consistent with an anorthositic protolith, probably a mixture of nonpristine “anorthositic gabbros” and pristine anorthosites, anorthositic norites, norites, troctolites, and possibly dunites. The source protolith of the VHA rocks apparently had a significant KREEP component and the protolith for Apollo 16 KREEP must have had nearly the same Rb/Sr ratio as that of Apollo 14 KREEP. Two‐stage model calculations using the (T,ISr) parameters suggest that some of the melt rocks were open Rb‐Sr systems during impact melting. Rubidium‐strontium model ages for the FM and VHA rocks show a smoothly monotonic decrease with increasing Rb content. Model ages of the ANMR are uniformly low relative to 4.56 b.y. It is suggested that these effects are due to formation of these rocks by impact partial melting. Rubidium‐strontium model ages of the KREEP rocks do not vary with Rb content, suggesting that they were closed Rb‐Sr systems during impact melting. Samarium‐neodymium data for mineral separates of 67747 showed only a small variation in Sm/Nd ratio and yield an imprecisely defined isochron of 3.6 ± 0.4 b.y. Plagioclase separates of 67559 appear not to have reached Nd isotopic equilibrium during impact melting; consequently, the internal isochron age is also imprecisely defined at 4.0 ± 0.4 b.y. ϵNd values for the melt rocks are about ‐1 to ‐2 at their Rb‐Sr or 40Ar‐39Ar ages, are consistent with the protolith compositions inferred from the Rb‐Sr data, and show that lunar crustal Sm‐Nd evolution departed from chondritic evolution early in lunar history. The average chondritic uniform reservoir (CHUR) model age of those ANMR and FM rocks that apparently remained closed Rb‐Sr and Sm‐Nd systems during impact melting is 4.37 ± 0.10 b.y., consistent with earlier suggestions that the lunar crust formed at about that time. The average CHUR model age of two VHA rocks is 4.10 ± 0.15 b.y., suggesting that these rocks were open Sm‐Nd systems as well as open Rb‐Sr systems during impact (partial?) melting. Current hypotheses for the site geology, stratigraphy, and radiometric age data lead us to suggest that the Nectaris basin and the Descartes formation were formed about 4.1 b.y. ago, and the Imbrium basin and the Cayley formation were formed about 3.81–3.86 b.y. ago. The ages of some individual melt rocks probably date “local” cratering events, however.